1. Field of the Invention
The present invention relates to a polishing method and apparatus, and more particularly to a polishing method and apparatus for chemical mechanical polishing a copper (Cu) layer formed on a substrate such as a semiconductor wafer and then cleaning the polished substrate.
2. Description of the Related Art
Conventionally, in order to form a wiring circuit on a semiconductor substrate, a conductive film is deposited over a surface of a substrate by a sputtering process or the like, and then unnecessary portions are removed from the conductive film by a chemical dry etching process using a photoresist for a mask pattern.
Generally, aluminum or aluminum alloy has been used as a material for forming a wiring circuit. However, the higher integration of integrated circuits on the semiconductor substrate in recent years requires the narrower wiring to thus increase the current density, resulting in generating thermal stress in the wiring and increasing the temperature of the wiring. This unfavorable condition becomes more significant, as wiring material such as aluminum is thinner due to stress-migration or electromigration, finally causing a breaking of wire or a short circuit.
Hence, in order to prevent the wiring from generating excess heat while current flows, a material such as copper having a higher electrical conductivity is required to be used for a wiring circuit. However, since copper or copper alloy is not suited for the dry etching process, it is difficult to adopt the above-mentioned method in which the wiring pattern is formed after depositing the conductive film over the whole surface of the substrate. Therefore, one possible process is that grooves for a wiring circuit having a predetermined pattern are formed, and then the grooves are filled with copper or copper alloy. This process eliminates the etching process of removing unnecessary portions of the film, and needs only a polishing process of removing unevenness or irregularities of the surface. Further, this process offers the advantages that portions called wiring holes connecting between an upper layer and a lower layer in a multilayer circuit can be formed at the same time.
However, as the width of the wiring is narrower, such wiring grooves or wiring holes have a considerably higher aspect ratio (the ratio of depth to diameter or width), and hence it is difficult to fill the grooves or the holes with metal uniformly by the sputtering process. Further, although a chemical vapor deposition (CVD) process is used to deposit various materials, it is difficult to prepare an appropriate gas material for copper or copper alloy, and if an organic material is used for depositing copper or copper alloy, carbon (C) is mixed into a deposited film to increase migration of the film.
Therefore, there has been proposed a method in which a substrate is dipped in a plating solution to plate the substrate with copper by an electrolytic plating or an electroless plating and then an unnecessary portion of a copper layer is removed from the substrate by a chemical mechanical polishing (CMP) process. This formation of film or layer by the plating allows wiring grooves having a high aspect ratio to be uniformly filled with a metal having a high electrical conductivity. In the CMP process, a semiconductor wafer held by the top ring is pressed against a polishing cloth attached to a turntable, while supplying a polishing liquid containing abrasive particles, and thus the copper layer on the semiconductor wafer is polished.
Immediately after the copper layer is polished in the CMP process, a polished surface of the copper layer on the semiconductor wafer has a high activity so that the polished surface is liable to be oxidized. If the polished surface on the semiconductor wafer is left as it is, then an oxide film is formed by natural oxidation on the polished surface of the semiconductor wafer. However, such oxide film tends to be formed irregularly or nonuniformly because no control is made of formation of the oxide film, and hence the formed oxide film is of poor quality. If the oxide film is left as it is, then oxidation of the polished surface of the semiconductor wafer is further developed. Particularly, in the case where copper is used as a material for forming a wiring circuit of a semiconductor device, electrical characteristics are changed to produce products inferior in quality.
Further, during polishing, a polishing liquid or by-product generated by polishing reaches the back surface opposite to the polished surface of the semiconductor wafer and is attached thereto, and may possibly contaminate the atmosphere in a clean room.